1. Field of the Invention
The present invention relates to Orthogonal Frequency Division Multiplexing (OFDM) modulation methods for wireless communications systems, and more particularly to a method for mitigating interference in OFDM communications systems that provides a modified pilot signal design to mitigate interference in high Doppler effect (mobile) environments.
2. Description of the Related Art
An OFDM signal includes a number of independently modulated, mutually orthogonal subcarriers over which large number of signals can be transmitted over a similar time period. This allows for very effective use of the spectrum with high bandwidth efficiency. Many digital communication standards have selected OFDM as their modulation scheme of choice. Because OFDM is known to be one of the most efficient modulation methods available, it is desirable to use OFDM widely, including in a mobile environment. Such mobile environments often have high Doppler effect shifting of frequencies because of the relative speed differences between communicating transmitters and receivers. High data rate communication systems inherently suffer from frequency selectivity, which causes Inter Symbol Interference (ISI) in a high Doppler environment.
To mitigate the effect of ISI, OFDM systems use a Cyclic Prefix (CP) of length greater than or equal to the channel length. At the receiver, this CP is discarded to recover the ISI-free OFDM symbol. The CP decouples the subcarriers of the OFDM symbol, making it possible to use a single-tap equalizer at the receiver, thus simplifying the receiver design.
This ability of OFDM to allow high-speed data transmission has led to its adaptation in many broadband standards, including Digital Audio and Video Broadcasting (DAB, DVB), wireless local area network (WLAN) standards (e.g., IEEE 802.11a/b/g and HIPERLAN/2), high-speed transmission over digital subscriber lines (DSL) and the Digital Terrestrial/Television Multimedia Broadcasting (DTMB) standard. A number of emerging broadband wireless communication standards are using or planning to use OFDM modulation including 802.16 (WiMAX), 802.20 Mobile Wireless Broadband Access (MWBA) and other emerging cellular wireless communication systems like 3GPP evolution and 4G.
OFDM systems depend upon orthogonality of the subcarriers. If orthogonality is lost, the information on one subcarrier is leaked into other adjacent subcarriers, i.e., the subcarriers are no longer decoupled. This leakage is termed inter-carrier interference (ICI). There are three main contributing factors to ICI: namely, phase noise, frequency error, and Doppler shift. In practice, the effect of phase noise and frequency error can be minimized by proper receiver design, and thus these two factors do not amount for a large ICI component in well-designed systems. Doppler shift appears due to the relative motion of the transmitter and receiver and is the main cause of ICI, especially in mobile wireless environments where the channel is continuously changing with time. Under such conditions, maintaining the orthogonality of OFDM subcarriers is a challenge, particularly if the time variation is large.
A direct consequence of operating OFDM systems in a high-Doppler environment is the introduction of ICI. The effect of ICI is that the subcarriers are no longer orthogonal. This results in energy leakage from a subcarrier to its neighbors, i.e., the subcarriers are no longer decoupled. The presence of ICI makes the task of channel estimation more challenging, since now both the frequency response at the subcarrier and also the interference caused by its neighbors have to be estimated.
Recently, a frequency domain high-performance computationally efficient OFDM channel estimation algorithm in the presence of severe ICI was proposed by the inventors in U.S. patent application Ser. No. 12/588,585.
Many OFDM-based systems use pilot subcarriers for channel estimation and tracking. For fixed and slowly varying channels, the optimum pilot pattern consists of equally spaced individual pilot subcarriers. However, the same pilot patterns are no longer optimal when ICI is introduced in a high Doppler environment.
Thus, a method for mitigating interference in OFDM communications systems solving the aforementioned problems is desired.